Senile macular degeneration is a leading cause of blindness in the United States, and presumed ocular histoplasmosis syndrome, while less common, is an important cause of blindness. Although various treatments have been proposed for each of these conditions, including photocoagulation, no treatment has been shown to be of benefit for either condition in a controlled randomized trial with a sufficient number of patients. The Macular Photocoagulation Study is a controlled clinical trial supported by the National Eye Institute designed to evaluate the efficiency of argon laser photocoagulation in preserving vision in patients with senile maculopathy and in patients with ocular histoplasmosis. The eyes of patients eligible for photocoagulation treatment will be randomly assigned to treatment with argon laser photocoagulation and no treatment. Each clinic will recruit approximately 100 patients. All patients will be followed for five years under a detailed protocol. The results will be examined at six-month intervals by Data Monitoring Committee.